I apologize if this has been covered before of has been implemented already. I am using an older version and will not get the newest one till, well, a month from now at the earliest.
Anyhow, it would be convenient to click a button and have calibre make changes I make to books on the computer get reflected on the reader. The biggest thing for me is if I change the books data (ie author, book name) it can be synched with the reader vice doing it on both.

Hell, even a sync like how iTunes works with the iPod. I add book to library, it gets added to reader, I take it off computer, it goes away from reader.

So I went and edited information on my library... almost all of them actually since I finally figured out how to get them to sort correctly (only 85 books right now)
Well, I selected them all and clicked send to device. I ended up with a total of 170 books on the device. Ooops. I must have done something wrong eh.

I didnt try that. After I ended up with double the books on the device I removed the books on the device and resend them and now it is good to go. I downloaded pictures of some book covers today so I am going to try to change it on my Mac and then send to device to see if I get 2 copies of the same book again.

So I tried it again. I edited the information. Turns out I mislabeled the author. I fixed it and clicked send to device. I ended up with the same book twice. One under the wrong author and the right author...